Transition-metal-catalyzed C–H bond activation as a sustainable strategy for the synthesis of fluorinated molecules: an overview

The last decade has witnessed the emergence of innovative synthetic tools for the synthesis of fluorinated molecules. Among these approaches, the transition-metal-catalyzed functionalization of various scaffolds with a panel of fluorinated groups (XRF, X = S, Se, O) offered straightforward access to high value-added compounds. This review will highlight the main advances made in the field with the transition-metal-catalyzed functionalization of C(sp2) and C(sp3) centers with SCF3, SeCF3, or OCH2CF3 groups among others, by C–H bond activation. The scope and limitations of these transformations are discussed in this review.


Introduction
Nowadays, fluorinated molecules represent an indispensable class of molecules in chemistry and in chemical biology. Thanks to the unique properties of the fluorine atom or fluorinated groups to modulate biological and physicochemical properties of the parent non-fluorinated molecules [1,2], their incorporation in various scaffolds afforded high value-added compounds as demonstrated by their numerous applications in the industrial sector such as drugs, agrochemicals, and materials. To further illustrate the ubiquity of fluorinated compounds, almost 20% of pharmaceuticals and 30-40% of agrochemicals [3][4][5] contain at least one fluorine atom. Because of the exceptional features of fluorinated derivatives, tremendous develop-ments and discoveries have been made in this blossoming research area, with a high interest for fundamental research as well as the industry [6][7][8][9][10][11]. Among them [2,5,[12][13][14][15][16][17][18], the direct functionalization of a simple C-H bond by transitionmetal catalysis  became an important tool offering new retrosynthetic disconnections. In this context, a strong interest from the scientific community was shown towards the challenging synthesis of fluorinated molecules by transition-metalcatalyzed C-H bond activation [44][45][46][47][48][49][50], allowing the functionalization of complex molecules and even for late-stage functionalization strategy [51][52][53] for the synthesis of natural products [42,[54][55][56][57][58][59]. Scheme 2: Cu(OAc) 2 -promoted mono-and ditrifluoromethylthiolation of benzamide derivatives derived from 8-aminoquinoline reported by the group of Daugulis [114].
The goal of this review is to highlight and discuss the recent approaches for the synthesis of fluorinated derivatives by the direct incorporation of a fluorinated group of XR F type (e.g., SCF 3 , SeCF 3 , SCF 2 CO 2 Et, OCH 2 CF 3 ) by transition-metal-catalyzed C-H bond activation (Scheme 1). The review will be organized in two main parts, dedicated to the construction of a C-SCF 2 R/SeCF 3 and C-OCH 2 CF 3 bond. This review does not aim to be exhaustive and key examples were carefully chosen to provide the reader a nice overview. Since reviews dealing with transition-metal-catalyzed functionalization of compounds by C-H bond activation with fluorinated reagents [60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77] and vinylic, allylic or propargylic fluorinated building blocks [49] have been already reported, these reactions will not be included.
Scheme 1: Transition-metal-catalyzed C-XR F bond formation by C-H bond activation: an overview. Review I. Transition-metal-catalyzed directed C-chalcogen bond formation (C-S, C-Se) by C-H bond activation In the past decade, particular attention has been paid to the development of new methodologies for the incorporation of sulfur-containing fluorinated groups. Although the transitionmetal-catalyzed direct C-H bond functionalization appeared to be a powerful tool for the installation of C-C, C-N, or C-O bonds, the direct formation of a C(sp 2 )-SR F or a C(sp 3 )-SR F bond remains a challenging task. In this context, key players in the field have been interested in the design of original methodologies for the trifluoromethylthiolation and more recently the difluoromethylthiolation of various compounds by transitionmetal catalysis [78]. Moreover, a recent interest was devoted to the trifluoromethylselenolation reaction as depicted in this section.
I.1) Transition-metal-catalyzed C-H trifluoromethylthiolation of aromatic C(sp 2 ) centers Scheme 3: Trifluoromethylthiolation of azacalix[1]arene [3]pyridines using copper salts and a nucleophilic SCF 3 source reported by Wang and co-workers [100]. a A mixture of CHCl 3 /MeOH 1:1 was used as solvent. electron-withdrawing groups (1d) were difunctionalized in good yields. The substitution pattern on the aromatic ring did not affect the reaction efficiency, the meta-substituted derivative 2e as well as the ortho-substituted derivative 2f were obtained in high yields (70% and 63% yields, respectively). It should be noted that the presence of ortho-substituents on the aryl residue allowed the monofunctionalization to occur selectively. Also, amide 1g bearing a disubstituted arene was successfully functionalized in 59% yield. Finally, the difunctionalized thiophene derivative 2h was obtained in 56% yield.
Palladium catalysis: Several works have been reported for the palladium-catalyzed trifluoromethylthiolation reaction of various aromatic compounds 5 by C-H bond activation and involved in most cases an electrophilic SCF 3 source (R 1 R 2 NSCF 3 ). For these transformations, the following working hypothesis was generally suggested (Scheme 4). After coordination of the palladium catalyst to a directing group, the Scheme 4: Working hypothesis for the palladium-catalyzed C-H trifluoromethylthiolation reaction.
metallacycle A is formed. This latter undergoes an oxidative addition in the presence of an electrophilic source or an oxidation/ ligand exchange in the presence of a nucleophilic source (i.e., AgSCF 3 ) and an oxidant (B in Scheme 4). Finally, after a reductive elimination step, the expected functionalized product 6 is obtained and the palladium catalyst is regenerated.
Scheme 5: Trifluoromethylthiolation of 2-arylpyridine derivatives and analogs by means of palladium-catalyzed C-H activation reported by the group of Shen [117]. a 6 equiv of I were used at 150 °C for 24 h. b The reaction was conducted at 150 °C for 24 h.
In 2014, Shen and Xu [117] developed a new methodology for the selective functionalization of 2-arylpyridine derivatives using an electrophilic SCF 3 reagent, the Haas reagent I (Scheme 5) [118]. A broad range of 2-arylpyridine derivatives were trifluoromethylthiolated in good to high yields (18 examples, from 52 to 91% yields). The substitution pattern of the aromatic ring had no impact on the outcome of the reaction as illustrated with substrates substituted by a methyl group (7b, 7d, and 7f) at the para-, meta-and ortho-positions, which were readily functionalized in 71%, 84% and 78% yields, respectively. This reaction was also tolerant of a 2-naphthyl group, the palladium-catalyzed trifluromethylthiolation afforded the corresponding product 8h in 76% yield. Also, the 2,4-dimethoxylated substrate 7g and the benzothiophene derivative 7i were successfully trifluoromethylthiolated in 76% and 63% yields, respectively. This reaction proved to be compatible with the presence of an ester (8c) or a halogen (8e). Other directing groups, such as substituted pyridines (9a and 9b) and pyrimidine (9c) turned out to be also efficient in this transformation (Scheme 5, 4 examples, up to 84% yield).
In this study, two mechanisms were reported. The first one suggested that a palladacycle C is formed after the irreversible chelation of the 2-phenylpyridine substrate with palladium, which is the rate-determining step (KIE = 2.7). Subsequently palladacycle C is oxidized by Selectfluor ® to form a palladium(IV) complex D. After a ligand exchange with AgSCF 3 , the intermediate E is obtained, which, after reductive elimination, releases the desired product 12 and regenerates the catalyst. Alternatively, a ligand exchange with AgSCF 3 occurs before the oxidation step, generating the palladium(II) complex F. After an oxidative addition in the presence of Selectfluor ® , the palladium(IV) intermediate E is generated. Finally, after reductive elimination step, the desired product 12 is released and the catalyst regenerated. Note that, in this process, Selectfluor ® is playing a key role. Indeed, using this electrophilic fluorinating source as oxidant generates a Pd(IV)(ppy)F(OAc) 2 (ppy = 2-phenylpyridine) complex as intermediate. As the competitive C-F bond formation was disfavored (slow reductive elimination step), the desired trifluoromethylthiolated product 12 is selectively afforded after a F/SCF 3 ligand exchange.
In 2018, Anbarasan and co-workers described a palladium-catalyzed trifluoromethylthiolation of arenes by C-H bond activation bearing several directing groups (Scheme 8) [121]. With Scheme 8: Palladium-catalyzed ortho-trifluoromethylthiolation of 2-arylpyridine and analogs reported by Anbarasan [121]. a The reaction was conducted at 120 °C using 1.1 equiv of Billard reagent IV (R = Me). this methodology, the functionalization of 2-phenylpyridine derivatives was possible (10 examples, up to 81% yield) using the Billard reagents III or IV [122]. The trifluoromethylthiolation of 2-arylpyridines substituted by electron-donating groups such as methyl, methoxy or methylthio groups (17b-d) or by halogen (17e) was achieved (Scheme 8, up to 77% yield). Note that in case of disubstituted 2-(4-ethoxy-3-fluorophenyl)pyridine (17h), the expected product 18h was isolated in 31% yield. Moreover, selective oxidation of the SCF 3 residue into the cor-responding sulfoxide and the sulfone was possible and the corresponding products were obtained in 98% and 95% yields, respectively.
The same year, Besset and co-workers reported a palladium-catalyzed C(sp 2 )-SCF 3 bond formation on amides derived from 8-aminoquinoline as a cleavable directing group in the presence of the Munavalli reagent V (Scheme 9, 12 examples, up to 71% yield) [106]. Depending on the substitution pattern on the aromatic ring, the amides were mono-or difunctionalized. Indeed, meta-and ortho-substituted derivatives (21a-d) were selectively trifluoromethylthiolated while para-substituted substrates led to the difunctionalized products 22e and 22f. Within these reaction conditions, the polysubstituted derivative 21g was also functionalized in high yield (71%).
Pleasingly, other metals have been also successfully applied for the trifluoromethylthiolation of aromatic derivatives by C(sp 2 )-H bond activation such as Rh(III) and Co(III)-based catalysts as depicted below.

Rhodium catalysis:
In 2015, the group of Li disclosed the Cp*Rh(III)-catalyzed regioselective trifluoromethylthiolation of N-substituted indoles with (substituted) pyridines or pyrimidine as the directing groups (Scheme 10) [123]. The selective tri-fluoromethylthiolation of indoles at the C2 position was achieved in the presence of N-(trifluoromethylthio)saccharine (VI, Shen's reagent) as both oxidant and electrophilic source (18 examples, up to 91% yield). Indoles bearing various electron-donating and electron-withdrawing groups as well as halogens at the C5-position and at the C6-position were functionalized in high yields (24a-f, 82-87% yields). The substitution of the indole at the C3-position did not impact the reaction and the product 24g was obtained in 91% yield. Substituted pyridines and pyrimidine (24h and 24i) were also used as directing groups (7 examples, up to 86% yield). This methodology was extended to the functionalization of other heteroaromatic derivatives (24j, 87% yield). It should be noted that the presence of zinc triflate, a Lewis acid, was used for the activation of the electrophilic source VI.
Cobalt catalysis: In 2017, Wang described the Cp*Co(III)-catalyzed trifluoromethylthiolation of 2-phenylpyridine derivatives using AgSCF 3 (Scheme 11) [124]. This methodology allowed the functionalization of several aromatic compounds bearing a pyridine or a pyrimidine as a directing group (20 examples, up to 65% yield). The reaction proceeded smoothly with substrates bearing an electron-donating group (25b,c), halogen (25d) or withdrawing group (25e) and the desired SCF 3 -containing products were obtained in moderate to good yields. The functionalization of trisubstituted arene 25g and Scheme 10: Regioselective Cp*Rh(III)-catalyzed directed trifluoromethylthiolation reported by the group of Li [123]. Cp* = pentamethylcyclopentadienyl.
heteroarene 25h was also possible leading to the corresponding products 26g and 26h in moderate yields (23% and 42% yields, respectively). Regarding the reaction mechanism, the active Co(III) complex G was obtained from the dimeric catalyst [Cp*CoI 2 ] 2 in the presence AgSCF 3 and/or NaOPiv·H 2 O. Then, the reversible formation of the metallacycle H occurs, which after a ligand exchange in the presence of AgSCF 3 leads to the formation of adduct J. The product 26 is released via a reductive elimination step, generating at the same time the reduced cobalt Cp*Co(I), which is converted to the active catalyst after oxidation.
The same year, Yoshino and Matsunaga described a similar methodology, using the cobalt(III) complex [Cp*Co(CH 3 CN) 3 ](SbF 6 ) 2 and N-trifluoromethylthiodibenzenesulfonimide VII as electrophilic SCF 3 source (Scheme 12) [125]. Under these reaction conditions, 2-arylpyridines (9 examples, up to 94% yield) or 6-arylpurines (10 examples, up to 72% yield) were ortho-trifluoromethylthiolated. The substitution pattern of the aryl part did not impact the outcome of the reaction. This methodology was also tolerant to a large range of functional groups (ester, halogen) as illustrated by the products 28c, 28j, 28d, 28k, and 28g.

I.2) Transition-metal-catalyzed C-H trifluoromethylthiolation of vinylic C(sp 2 ) centers
Several research groups have been interested in the development of strategies for the formation of vinylic C(sp 2 )-SCF 3 bonds, offering an efficient tool towards the synthesis of challenging Z-isomers.
Palladium catalysis: In 2018, Bouillon and Besset described the first example of a selective palladium-catalyzed trifluoromethylthiolation of α-arylacrylamides derived from 8-aminoquinolines 29 by C-H bond activation (Scheme 13) [104]. Using the Munavalli reagent V as an electrophilic SCF 3 source, this diastereoselective method selectively led to the formation of Z-isomers and turned out to be robust (not air or moisture sensitive). Under these mild reaction conditions, a panel of α-(hetero)arylacrylamides were trifluoromethylthiolated in good to high yields. Acrylamides substituted at the α-position by an aryl bearing an electron-donating group (OMe) or halogen (Cl) at the para-position were readily functionalized (30b and 30c, 70% and 75% yields, respectively). The substitution of the arene with a CF 3 residue at the meta-position or with a methoxy group at the ortho-position did not have any impact to the outcome of the reaction (30e and 30f, 62% and 78% yields, re-Scheme 11: Cp*Co(III)-catalyzed ortho-trifluoromethylthiolation of 2-phenylpyridine and 2-phenylpyrimidine derivatives reported by Wang and co-workers [124]. a CH 2 Cl 2 was used as solvent.
spectively). It should be noted that acrylamides bearing a disubstituted arene (29g,h) and an heteroaryl (29i) at the α-position were also suitable substrates for this reaction. Finally, acrylamides bearing a methyl group at the α-position (29j) or the α,β-dimethylated acrylamide (29k) were suitable substrates albeit the corresponding products were obtained in 30% and 20% yields, respectively. Several mechanistic experiments revealed that the C(sp 2 )-H bond activation step was reversible and represented the rate-determining step (KIE = 2.4). First, the chelation of the palladium(II) catalyst with the bidentate Scheme 12: Cp*Co(III)-catalyzed ortho-trifluoromethylthiolation of 2-phenylpyridine and 6-phenylpurine derivatives described by Yoshino and Matsunaga [125]. a Without AgSbF 6 . b 2.0 equiv of VII, 10 mol % of AgOAc and 30 mol % of Gd(OTf) 3 were used instead of AgSbF 6 . directing group, followed by the C(sp 2 )-H bond activation involving a concerted metalation-deprotonation pathway affords the metallacycle K. After an oxidative addition in the N-SCF 3 bond of the Munavalli reagent V, the palladium(IV) species L is obtained. Finally, the reductive elimination affords the product and regenerates the catalyst. The same year, Besset and co-workers extended this methodology to a larger class of acrylamides [106].

I.3) Transition-metal-catalyzed trifluoromethylthiolation of aliphatic C(sp 3 )-H bonds
Despite the important progresses presented in the previous section, some limitations of the trifluoromethylthiolation persist. In particular, the functionalization of a C(sp 3 )-H bond with a trifluoromethylthiolated moiety by transition-metal-catalyzed C-H activation remains a challenging task both in terms of reactivity and selectivity.
In 2015, Besset reported the first C(sp 3 )-SCF 3 bond formation of unactivated primary C(sp 3 ) centers by transition-metal-catalyzed C-H activation with the Munavalli or the Billard reagents as the trifluoromethylthiolation source (Scheme 14) [126]. Using a bidentate directing group, this methodology allowed the functionalization of a large range of aliphatic amides with a pri-mary β-C(sp 3 )-H bond (21 examples, up to 53% yield). The methodology was applied to the functionalization of a series of amides having an α-quaternary center (α,α-dialkyl (31a), α-alkyl,α-benzyl derivatives 31c-f) as well as to an amide with an α-tertiary center (31b) and pleasingly, the presence of α-C-H bonds did not have a significant impact on the outcome of the reaction. It should be noted that this methodology afforded the products with a high regioselectivity, and no incorporation of the SCF 3 moiety on the benzylic or at the C5 position of the quinoline part of the directing group was observed. Note that in 2018, Besset and Lebel developed a more efficient process for the palladium-catalyzed trifluoromethylthiolation by C-H bond activation under continuous flow conditions [127].

I.4) Difluoromethylthiolation of aromatic and vinylic C(sp 2 )-H bonds (C-SCF 2 H and C-SCF 2 CO 2 Et bonds)
Scheme 13: Diastereoselective trifluoromethylthiolation of acrylamide derivatives derived from 8-aminoquinoline using PdCl 2 reported by Bouillon and Besset [104,106]. a 20 mol % of PdCl 2 and 2.0 equiv of SCF 3 source V were used for 36 h. b The reaction was conducted with 30 mol % of PdCl 2 and 2.0 equiv of reagent V were used. [126]. Product 32d was contaminated with 10% of an inseparable impurity.

Scheme 14: C(sp 3 )-SCF 3 bond formation on aliphatic amide derivatives derived from 8-aminoquinoline by palladium-catalyzed C-H bond activation described by Besset and co-workers
In 2022, He and Pan reported the first example of a difluoromethylthiolation achieved by transition-metal-catalyzed C-H bond activation (Scheme 15) [137]. Using the reagent IX and a catalytic amount of PdCl 2 , they succeeded in the functionalization of acrylamides 33 derived from 8-aminoquinoline. Within these mild conditions, α-arylacrylamides substituted at para-, meta-, and ortho-positions were readily difluoromethylthiolated (34a-g, 81-95% yields). This reaction was also tolerant to a large class of functional groups (halogens, cyano, trifluoromethyl), affording products 34c,d and 34e in high yields (86%, 89% and 81%, respectively). The α-methyl-substituted acrylamide 33j also underwent difluoromethylthiolation to give product 34j in 81% isolated yield. The α,β-and β-substituted acrylamides were functionalized in high yields (34h,i and 34k-m, 71-86%). The plausible mechanism is similar as the one reported by Besset for the trifluoromethylthiolation of acrylamides derived from 8-aminoquinoline (Scheme 13).
The same year, Besset and co-workers reported the first palladium-catalyzed C(sp 2 )-SCF 2 CO 2 Et bond formation by C-H bond activation (Scheme 16) [138]. In the presence of the electrophilic SCF 2 CO 2 Et source X, the methodology was successfully applied for the functionalization of 2-arylpyridine derivatives 35a-i as well as 2-vinylpyridine derivatives 35j-m (35 examples, up to 87% yield). The substitution pattern of the aryl substituent of the 2-phenylpyridine derivatives did not influence the reaction as for instance products 36b, 36h, and 36i were obtained in good yields (70%, 63% and 61% yield, respectively). This reaction was tolerant to a broad range of functional groups such as halogens, ester, aldehyde, cyano, and nitro (36c-g, 36-74% yield). It is noteworthy that a disubstituted compound 35j and a thiophene derivative 35k were also efficiently difluoromethylthiolated (36j and 36k, 72% and 65%, respectively). α-Substituted vinylpyridines with electron-donating or electron-withdrawing groups on the aromatic ring were functionalized and 36l and 36m were easily isolated in 73% and 81% yields, respectively. Even an α,β-disubstituted vinylpyridine 35n and the benzoquinoline 35o were smoothly functionalized showing the efficiency of the approach. Of high interest, the modularity of the SCF 2 CO 2 Et was highlighted by its conversion into various other fluorinated residues (amide, carboxylic acid) and its selective oxidation into the corresponding sulfoxide and sulfone.

I.5) Trifluoromethylselenolation of aromatic and vinylic C(sp 2 )-H bonds by palladium catalysis
Very recently, the palladium-catalyzed trifluoromethylselenolation of (hetero)aromatic and olefinic derivatives has been investigated by the group of Billard using similar catalytic systems as those depicted for the trifluoromethylthiolation reactions. Indeed, using amides 37 derived from 5-methoxy-8-aminoquinoline, the functionalization of (hetero)aromatic compounds was achieved using 20 mol % of Pd(CH 3 CN) 4 Cl 2 in the presence of TolSO 2 SeCF 3 as the fluorinating source (14 examples, Scheme 17) [139]. Of note, when the reaction was carried out on derivatives bearing substituents at the meta-(37c and 37d) as well as at the para-(37a and 37b) positions, the corresponding products 38a-d were obtained as a mixture of mono-and disubstituted trifluoromethylselenolated derivatives with global yields ranging from 48% to 70%. When one of the ortho-posi-Scheme 15: Regio-and diastereoselective difluoromethylthiolation of acrylamides under palladium catalysis reported by He and Pan [137].
tions of the aromatic ring was substituted with a Me (37e), a OMe (37f), a Cl (37g) or a CF 3 (37h) group, the corresponding compounds 38e, 38f, 38g, and 38h were isolated in moderate to high yields (43% to 80%). The methodology also allowed the trifluoromethylselenolation of the furan derivative 37i, which led to the desired product 38i in 30% yield. A careful monitoring of the reaction unveiled the rapid formation of the CF 3 SeSeCF 3 dimer, which could be the active trifluoromethylselenolating reagent in this transformation.
Furthermore, a series of β-substituted acrylamides 39i-m with various substituents readily underwent the trifluoromethylselenolation reaction with high selectivity in moderate yields. Finally, the SeCF 3 -containing β-methylacrylamide 40n was also obtained in 37% yield.
II. Transition-metal-catalyzed fluoroalkoxylation of (hetero)arenes by C-H bond activation
In 2004, Sanford and co-workers reported the dehydrogenative 2,2,2-trifluoroethoxylation of benzo[h]quinoline under palladium catalysis in the presence of PhI(OAc) 2 as oxidant (Scheme 19) [161]. Since this seminal work and in the course of their investigation towards the development of new methods for the alkoxylation of C(sp 2 ) centers by transition-metal catalysis, few examples of transition-metal-catalyzed dehydrogenative 2,2,2-trifluoroethoxylation reactions have been reported. In 2021, the palladium-catalyzed ortho-2,2,2-trifluoroethoxylation of 3-arylcoumarins was depicted by the group of Kumar (6 examples, up to 69% yield) [162]. Further developments unveiled the use of copper catalysts for such functionalization. In 2013, the group of Daugulis described the copper-catalyzed ortho-2,2,2-trifluoroethoxylation of a 3-trifluoromethylated benzamide derived from 8-aminoquinoline, giving the corresponding product in 73% yield [149]. The group of Baidya showed that the dehydrogenative 2,2,2-trifluoroethoxylation of benzamide with another bidentate directing group was also possible in the presence of Cu(OAc) 2 and hexamethyldisilane [163]. Using N,N-and N,O-bidentate directing groups, the construction of C(sp 2 )-OCH 2 CF 3 bonds by C-H bond activation was also reported using Ni [164] and Co catalysis [165][166][167]. In 2022, Volla and co-workers reported the ortho-2,2,2-trifluoroethoxylation of benzamide using an N,O-bidentate directing group by merging Co-and visible light organophotocatalysis [168].
nalization of diversely substituted derivatives bearing either electron-donating groups (41b, 41d, and 41f) and electron-withdrawing groups (41c, 41e, and 41g) was achieved (19 examples, up to 93% yield). Of note, the transformation was also efficient with disubstituted substrates such as 41h-j. The authors suggested the following mechanism. After formation of the metallacycle O, the latter is oxidized leading to the Pd(IV) species P. After a ligand exchange, the intermediate Q is generated. Finally, a reductive elimination step affords the expected functionalized product 42 and regenerates the catalyst.
The 2,2,2-trifluoroethoxylation reaction is not restricted to amides. In 2020, Yorimitsu and co-workers developed a methodology allowing the formation of a C(sp 2 )-OR F bond through palladium-catalyzed C-H bond activation [169]. Using Pd(OPiv) 2 as the catalyst in the presence of PhI(OAc) 2 (PIDA), the naphthalene sulfoxide 43a was 2,2,2-trifluoroethoxylated in 82% yield (Scheme 21). The substituent of the sulfoxide part does not impact the efficiency of the reaction as illustrated by the synthesis of compounds 44b and 44c. The presence of an electron-donating substituent in the para-position of the directing group was found to be deleterious for the reaction since 44d was obtained in 31% yield while its brominated analog 44e was isolated in 70% yield. Mechanistic studies indicated that the C-H bond activation event was the rate-limiting step and the authors suggested a similar mechanism to the one depicted in Scheme 20: formation of a palladacycle thanks to a concerted metalation deprotonation (CMD) process followed by oxidation, ligand exchange with CF 3 CH 2 OH, and finally, reductive elimination affording the expected product and regenerating the catalyst. Gratifyingly, the approach was applied to the incorporation of other fluorinated moieties such as OCH 2 CF 2 H and OCH 2 (CF 2 ) n H (n = 2 or 4) and gave compounds 46a-c.
In 2021, Wang and co-workers described the selective palladium-catalyzed ortho-2,2,2-trifluoroethoxylation of a series of benzaldehydes (Scheme 22, 35 examples) using the amino acid ʟ-valine in the presence of K 2 S 2 O 8 and TFA at 80 °C [153]. This reaction proved to be highly tolerant to various substituents including a CF 3 group at the ortho-, meta-and para-positions (48m, 48j, and 48f, respectively), halogens (48d, 48h, and 48l), an ester moiety (48e and 48i), and a methoxy group (48c). Note that even di-and trisubstituted benzaldehydes 47n-q were smoothly functionalized under these conditions. The authors also suggested a plausible mechanism. The amino acid acts as an organocatalyst and first reacts with the benzaldehyde 47 to generate the transient directing group (47'). Then, formation of the palladacycle (species R) followed by its oxidation to a Pd(IV) intermediate and a ligand exchange with 2,2,2-trifluoroethanol leads to the formation of the species S. The latter complex S undergoes a reductive elimination leading to the compound 48' along with the regeneration of the palladium catalyst. Finally, after hydrolysis of 48', the expected product 48 is afforded together with the organocatalyst. Then, the group of Sun and Wang used a similar approach for the 2,2,2-trifluoroethoxylation of benzaldehydes under palladium catalysis using the amino acid 51 as organic catalyst in the presence of the fluoropyridinium salt 52 (19 examples, up to 88% yield, Scheme 23) [183]. Pleasingly, the methodology was extended to the formation of C(sp 2 )-OR F bonds starting from benzaldehyde (OR F = 2,2-difluoroethoxy 50g, 2,2-difluoropropoxy 50h, and 1,1,1,3,3,3-hexafluoroisopropoxy 50i).

Conclusion
In summary, this review provides an overview of the major developments made over the last years for the synthesis of fluorinated compounds by transition-metal-catalyzed C-H bond activation. This review focused on the construction of C(sp 2 )-XR F bonds and C(sp 3 )-XR F bonds with an emphasis on the trifluoro-methylation reaction and transformations using emergent fluorinated residues (SCF 3 , SeCF 3 , SCF 2 H, SCF 2 CO 2 Et or OCH 2 CF 3 groups). Well-designed catalytic systems and suitable fluorinating sources were the key of success for these major developments.
Despite these advances, synthetic challenges still need to be overcome. These synthetic tools are so far still restricted to some fluorinated moieties and extension to other high valueadded fluorinated residues [131,136,[193][194][195][196][197][198][199][200][201][202][203][204][205][206][207][208] is of high importance. Besides, in comparison with the functionalization of C(sp 2 ) centers on aromatic and vinylic derivatives, transitionmetal-catalyzed functionalization of C(sp 3 )-H bonds remains largely underexplored to date. Furthermore, the development of enantioselective transformations allowing the synthesis of enantioenriched fluorine-containing compounds by transitionmetal-catalyzed C-H bond activation will have a significant impact as for instance an access to pharmaceutically relevant derivatives. Finally, the use of abundant non-noble transition metals [209][210][211] in such reactions combined or not with modern technologies (photocatalysis and electrocatalysis) is still underexplored and any advances will be of high importance especially from a sustainability point of view aiming at developing greener synthetic routes towards fluorinated molecules. Scheme 24: Pd(II)-catalyzed selective 2,2,2-trifluoroethoxylation of aliphatic amides using a bidentate directing group reported by the groups of Chen [71] and Shi [192].